This invention related to ultrasonic surgical tools and method and also relates to associated manufacturing techniques.
Ultrasonic cutting is widely used in industrial and food processing applications to produce a clean and accurate cut. However, it is still at the doorsteps of worldwide acceptance as a tool in surgical applications, mainly due to the difficulties with deliverability to complex locations and to the high temperatures that can be generated at the cut site.
Ultrasonic bone cutting devices inherit their operating principle from industrial ultrasound cutting devices. The principle schematic of such device is shown in FIG. 1. In response to a signal from a waveform generator 1, an electromechanical transducer 2 produces an ultrasonic standing wave in a horn or concentrator 3, ultrasonically vibrating a tip 3a to grind off sample material from a substrate 6 and to remove the ground material by a flowing abrasive mixture 4, where the substrate 6 is mounted to a ballast or anchor 8 via a compression spring 7. A similar process is used in ultrasound bone or tissue dissection surgery. However, ultrasound dissection of bones operates in a regime of unfavorable conditions.
Bone belongs to a third group of material classification (Kanevski, Acoust Phys. 7, 1-3, 1961) in which the resistance to shear is less than resistance to breakage: tp≡τp/σp<1, and plastic deformation dominates. In industrial ultrasound, cutting the third group materials is believed to be inefficient. In the materials of the first and second groups: tp>0.5, and the speed of cutting, νcut, is proportional to a product of square of amplitude of vibration, A, and applied pressure, P:νcut˜A2P. In the third group: νcut˜A√{square root over (P)}. Thus, in bone the speed of cutting cannot be substantially improved by having larger amplitude of vibration or applied pressure. Instead, the literature (e.g. L. D. Rosenberg, Physics and Technique of High Power Ultrasound, Volume 3, Chapter 2, USSR, Moscow, 1970) suggests that the design of the bone cutting ultrasound tip is important, implying that the pattern of tip vibration has also a defining critical effect on the bone cutting process efficiency and speed.